Call rcu_barrier() five times in the isc__mem_destroy()

Call rcu_barrier() five times in the isc__mem_destroy()

Because rcu_barrier() needs to be called as many times as the number of
nested call_rcu() calls (call_rcu() calls made from call_rcu thread),
and currently there's no mechanism to detect whether there are more
call_rcu callbacks scheduled, we simply call the rcu_barrier() multiple
times.  The overhead is negligible and it prevents rare assertion
failures caused by the check for memory leaks in isc__mem_destroy().